A meteorological radar makes it possible to locate precipitations such as rain, snow or hail, to measure their intensity and possibly to chart dangerous phenomena. Most meteorological radars are installed on the ground and often form part of a vaster meteorological monitoring network.
Nevertheless, ever more airborne applications are emerging, in order to make it possible to sidestep cumulonimbus formations. These enormous clouds are much feared by pilots as they sometimes produce violent storms. Specifically, lightning, hail, and strong wind shears inside the cloud are added to the risk of icing and may endanger the flight if the pilot tries to pass through.
A meteorological radar allows the detection of extended voluminal targets (objects) constituted by clouds, of which it must give the position, size and dangerousness. For this purpose, a meteorological radar can for example emit a wave in the X-band with the aid of its antenna. The distance to a cloud is deduced from the time necessary for the pulse emitted to perform the outward-return trip from the antenna of the radar to the cloud at the speed of light. This time corresponds simply to the duration between the emission of a pulse and the reception of its echo.
On the basis of the echoes received, radar images are formed. The pixels of each image are associated with reflectivity values corresponding to their amplitude. It is recalled that for a given point of the zone scanned by the radar, the reflectivity corresponds to the ratio of the energy reflected by this point to the total incident energy. The reflectivity value of each pixel is dependent on the intensity of the returned echo.
The size of a cloud is dependent on its surface area (the maximum horizontal distance over which it extends) and its elevation, (the maximum vertical distance over which it extends). The elevation defines first and foremost the dangerousness of the cloud: the higher a convective cloud, the more dangerous it is. But the dangerousness level of the cloud is also related to its reflectivity factor, denoted Z, which characterizes the concentration of hydrometeors in suspension in a volume of air, in liquid or solid form. Stated otherwise, the reflectivity factor Z represents the intensity of the cloud. On a logarithmic scale, it is measured in dBZ.
Specifically, a simplified representation of the clouds is displayed to the pilot by a viewing console, with the aid of a colour code characterizing the reflectivity, whether involving rain, snow or hail. For example, the colour black is often used for dry air, that is to say the absence of cloud. Green and yellow can be used for medium humidity concentrations. Red is often used for zones with very high humidity concentration, that is to say the most dangerous zones that absolutely must be sidestepped.
Currently, to determine the number of clouds, their position or else their intensity, the radar image is processed in one go, for example with the aid of a pixel-by-pixel processing procedure. By this processing, the reflectivity values associated with the pixels of the image are corrected, so as to approach as closely as possible to the real situation, that is to say the situation of the zone aimed at by the beam emitted by the radar antenna.
The quality of the result of this processing varies as a function of the separation distance between the zone and the radar antenna. Though it is relatively good at short distance, it becomes difficult to utilize at medium and long distance, especially if the resolution of the radar is low, this often being the case for airborne radars. In the case of meteorological radars, various phenomena will disturb the quality of the results.
For example, it then becomes particularly difficult to distinguish the ground clutter (spurious noise) from the useful signal, when the beam of the radar encompasses both the ground and the aerial domain.
Furthermore, the atmospheric attenuation makes it tricky to estimate the reflectivity levels.
All these constraints limit the maximum display distance to some hundred nautical miles, which in many situations is not sufficient.